Any ophthalmic lens intended to be held in a frame is associated to a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and with the position of the wearer's eyes relative to the frame.
In the most simple cases, the prescription is reduced to a positive or negative power prescription. The lens is termed unifocal and has a rotational symmetry. It is simply fitted in the frame in such a way that the wearer's main direction of glance coincides with the axis of symmetry of the lens.
For long-sighted wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far-vision power prescription and a near-vision power prescription. Lenses suitable for long-sighted wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. Nos. 5,270,745 or 5,272,495, FR-A-2-683 642, FR-A-2 699 294 or also FR-A-2 704 327. Progressive multifocal ophthalmic lenses include a far-vision zone, a near-vision zone, an intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. These lenses are all-purpose, in that they are suitable for wearers' differing day-to-day needs. Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition which corresponds to the power variation between the far-vision zone and the near-vision zone. More precisely, the addition, referenced A, corresponds to the power variation on the meridian between a point VL of the far-vision zone and a point VP of the near-vision zone, which are respectively called far-vision reference point and near-vision reference point, and which represent the points of intersection of the glance and the surface of the lens for far distance vision and for reading vision.
Conventionally, a lens can be defined by its base (or far-vision mean sphere) and by a power addition in the case of a multifocal lens. Starting from semi-finished lenses, only one face of which is formed with a given addition/base pair, it is possible to prepare lenses adapted to each wearer, by simple machining of a prescription face which is generally spherical or toric.
For any ophthalmic lens, the laws of the optics of ray tracings imply that optical defects appear when the light rays deviate from the central axis of any lens. These known defects which comprise inter alia a curvature defect or a power defect and an astigmatism defect can be called, in a generic way, obliquity defects of rays. A person skilled in the art knows how to compensate for these defects. For example, EP-A-0 990 939 proposes a method for determining, by optimization, an ophthalmic lens for a wearer having an astigmatism prescription. Obliquity defects have also been identified for progressive multifocal lenses. For example, WO-A-98 12590 describes a method for determining, by optimization, a set of multifocal ophthalmic lenses.
An ophthalmic lens comprises an optically useful central zone which can extend over the whole of the lens. Optically useful zone means a zone in which the curvature and astigmatism defects have been minimized to allow a visual comfort that is satisfactory for the wearer. On a progressive lens, the optically useful central zone will cover the far-vision zone, the near-vision zone and the progression zone.
Generally, the optically useful zone covers the whole of the lens which has a diameter of limited value. However, in some cases, a peripheral zone is provided on the periphery of the ophthalmic lens. This zone is termed peripheral because it does not meet the conditions of prescribed optical correction and has significant obliquity defects. The optical defects of the peripheral zone are not harmful to the wearer's visual comfort because this zone is situated outside of the wearer's field of view. A connection thus has to be provided between the optically useful central zone and the peripheral zone.
There are mainly two situations in which an ophthalmic lens will have such a peripheral zone. On the one hand, when the lens has a significant diameter which can be required by the shape of the frame, for example an elongated frame with a high curving contour, and on the other hand, when the power prescription is high, the lens thus having a significant edge or centre thickness which is desired to be reduced.
In the case of an ophthalmic lens intended to be fitted in a frame with curved contour, by 15° for example, the glass has a spherical or toric face with a high curvature (or base), between 6 diopters and 10 diopters, and a face calculated specifically to achieve the optimum correction of ametropia for the wearer in the optical centre and in the field of view. For example, for the same front face, having the same curvature, the rear face is machined to ensure the correction according to the ametropia of each wearer. The high curvature of the front face leads to a great thickness of the glass on the edges in the case of a negative lens or a great thickness of the glass in the centre in the case of a positive lens. These great thicknesses increase the weight of the lenses, which is detrimental to the wearer's comfort and makes the lenses unsightly. Moreover, for some frames, the edge thickness has to be limited to allow the glass to be fitted into the frame.
For negative lenses, the edge thicknesses can be reduced by planing with a manual facet. A thinning of the lens can also be controlled by optical optimization. An aspherization or an atorization can be calculated, at least for one of the faces of the lens, taking into account under the wearing conditions of the lens compared with a lens of the same prescription with a low curvature, in order to reduce the centre and edge thicknesses of the lens with a high curvature. Such solutions for optical aspherization or atorization are described for example in U.S. Pat. Nos. 6,698,884, 6,454,408, 6,334,681, 6,364,481 or also WO-A-97 35224.
In addition, in the case of a strong prescription lens, the cut-out lens has a significant edge thickness, on the nasal side for a hypermetropic positive lens and on the temporal side for a myopic negative lens. These extra thicknesses of the edges make it more complicated to fit the lens in the frame and make wearing the ophthalmic lenses heavier.
FR-A-2 638 246 proposes an ophthalmic lens with a high positive power and a reduced centre thickness. The lens described in this document has connecting zone between an optically useful central zone and a peripheral zone. This connecting zone is situated on the front face with a high curvature and has a rotational symmetry; the connection is obtained directly by moulding the front face.
EP-A-0 371 460 proposes an ophthalmic lens with a high power with a front face with rotational symmetry having a radius of curvature that increases over the periphery of the lens in order to reduce the edge thicknesses.
U.S. Pat. No. 6,176,577 proposes an ophthalmic lens for hypermetropia having a face with rotational symmetry having an optically useful central zone and a peripheral zone with reduced thickness. The connection between the central zone and the peripheral zone is obtained directly by moulding.